Modular filter systems, i.e., filter systems having a plurality of filter compartments or cells rather than a single large filter bed for a given unit, were primarily developed to reduce the size and complexity of the backwash or cleaning structure of single, large filter beds as well as obviate the need for a total system shutdown during the backwash or cleaning procedure. One such modular filter system is disclosed in U.S. Pat. No. 4,308,141 (“'141 patent”) the entire contents of which are incorporated herein by reference.
The '141 patent discloses a filter system that utilizes a tank or reservoir having a transversely positioned porous plate filter element vertically spaced above the floor or bottom of the reservoir. Wall partitions are used to divide the tank into a plurality of smaller individual compartments or cells that have a common underdrain and plenum chamber. Water containing undesired suspended particulate matter is introduced into the reservoir to filter the same. Gravity causes the influent to pass through the media in each of the cells or compartments. The filtered water or effluent is drawn away by a drain or effluent discharge located beneath or in the lower portion of the filter that communicates with the common plenum.
After a period of time of operating the modular filter system of the '141 patent to filter influent, the system requires cleaning or backwashing to remove the particulate matter collected in the media contained in the individual cells. Because the cells are backwashed sequentially, the filter system never needs to be shutdown for a backwash or a filter cleaning cycle and all of the filter cells except the one being cleaned are operable continuously.
The backwash system of the '141 patent includes a hydraulically isolating backwash hood assembly and associated carriage and platform support assembly. The backwash hood assembly depends from the mobile carriage and platform assembly and is initially centered over one of the six individual cells. Subsequently, the hood assembly is lowered to form a watertight seal with a particular cell to be backwashed. A backwash pump is activated creating a negative pressure within the backwash hood. Effluent created by adjacent individual cells is drawn up from the common underdrain through the media in the cell being backwashed to expand and wash the media. The effluent used to backwash a given cell is directed to waste through the hood assembly. This procedure is subsequently performed on all the remaining individual cells.
In the commercial application of the '141 patent, the water to be filtered by the modular filter is pre-treated by an extremely large pre-treatment system. The pre-treatment system includes a static mixer, flocculation paddles, settling tubes and a settling basin. The footprint of the pre-treatment system, i.e., the square footage of floor space occupied by the pretreatment system is approximately as large if not larger than the footprint of the corresponding modular filter. As a result, the entire filter system is extremely large, expensive and time consuming to construct. Obviously, these inherent disadvantages of the presently known modular filter systems severely limit the applications in which modular filter systems can be employed to filter water or wastewater. Further, the pre-treatment systems previously used in conjunction with modular filters process water at an extremely slow rate.